Among the various video display systems available in the art, an optical projection system is known to be capable of providing a high quality display in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodispiacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electrical signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of flight reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIG. 1, there is shown a cross sectional view of an array 10 of M.times.N thin film actuated mirrors 11 for use in an optical projection system, disclosed in a copending commonly owned application, U.S. Ser. No. 08/331,399, entitled "THIN FILM ACTUATED MIRROR ARRAY AND METHOD FOR THE MANUFACTURE THEREOF", comprising an active matrix 12, an array 13 of M.times.N thin film actuating structures 14, an array 15 of M.times.N supporting members 16 and an array 17 of M.times.N mirrors 18.
The active matrix 12 includes a substrate 19, an array of M.times.N transistors (not shown) and an array 20 of M.times.N connecting terminals 21. Each of the actuating structures 14 in the array 10 is provided with at least a thin film layer 22 of an electrodisplacive material such as a piezoelectric material, e.g., lead zirconium titanate(PZT) or an electrostrictive material, e.g., lead magnesium niobate(PMN), a first electrode 23, a second electrode 24 and an elastic layer 25 made of a ceramic, wherein the first and second electrodes 23, 24 are placed on top and bottom of the electrodisplacive layer 22, and the elastic layer 25 is placed on bottom of the second electrode 24, respectively. Each of the supporting member 16 is used for holding each of the actuating structures 14 in place by cantilevering each of the actuating structures 14 and for electrically connecting each of the actuating structures 14 and the active matrix 12 by being provided with a conduit 26 made of a metal, e.g., tungsten(W). Furthermore, each of the mirrors 18, made of a light reflecting material, e.g., aluminum(Al), is placed on top of each of the actuating structures 14.
In the thin film actuated mirror array 10, an electrical signal is applied across the electrodisplacive layer 22 located between the first and second electrodes 23, 24 in each of the actuating structures 14, causing a deformation thereof, which will, in turn, deform the mirror 18 placed on top thereof, thereby changing the optical path of the incident light beam.
There are a number of problems associated with the above-described array 10 of thin film actuated mirrors 11. The first and foremost to be mentioned, is the reduction in the optical efficiency of the array 10 due to the stress developed between the electrodisplacive layer 22 and the elastic layer 25 in each of the actuated mirrors 11 therein during the operation thereof. The stress developed therebetween will cause cracks to be formed on the mirror 18 and the first electrode 23 or will, in some cases, even force the mirror 18 and the first electrode 23 to be peeled off from each of the actuated mirrors 11, and, furthermore, the stress will force the electrodisplacive layer 22, and hence the mirror 18 and the first electrode 23 attached on top thereof, to bend in an irregular manner, all of which will contribute to the reducing of the optical efficiency of the array 10.